Hi Siobhan,
VSP4850 is not a stackable switch - so I presume you mean clustered with another switch.
The answer is yes you can downgrade it 100%, but as I recall you might have to downgrade to release 5.0 first because I think it will fail if you try directly (will not hurt the switch - so try - but go to 5.0 first if it does not take it).
I would note through that there have been many fixes since 4.2.3 - and also 4.2.3 is out of support, so you might be better thinking about upgrading the others instead....
Many Thanks,
Matt